At the very beginning of terms we have the unstructured variables
and atomic elements. The variables have already been defined as a
kind of words. Atomic elements are now atoms, numbers, negative
numbers and references. The names, the empty sets and the empty
lists are now found among the atoms. Among the structured terms we
will have compounds, lists and expressions:

X % is a variable and thus a termfoobar % is a name and thus a term3.1415 % is a number and thus a term-3 % is a negative number and thus a term- 3 % is an expression, corresponds to -(3)

The grammatical production
rules for terms can
be viewed as stratified into levels. This is used later for
parsing
operator
expressions. The levels range from 0 to 1200. Reading a term
usually
starts
with level 1200. To disambiguate from operator expressions a
negative
number is only recognized when the minus ("-") directly precedes
the
first digit of the number.